Liquids are dispersed in a gas in many process engineering installations. It is often of decisive importance for the liquid to be sprayed in the form of very fine droplets. The finer the droplets, the greater the specific droplet surface, which can lead to significant process engineering advantages. Thus, e.g. the size of a reaction vessel and its manufacturing costs are significantly dependent on the average droplet size. However, in many cases it is not adequate for the average droplet size to drop below a specific limit value. Considerable operating problems can in fact arise with a few significantly larger droplets. This is particularly the case if, as a result of their size, the droplets do not evaporate quickly enough, so that droplets or also pasty particles are deposited in subsequent components, e.g. on fabric filter bags or on fan blades and can lead to operating problems caused by incrustations or corrosion.
In order to finely spray liquids, use must be made either of high pressure single-fluid nozzles or medium pressure two-fluid nozzles. An advantage of two-fluid nozzles is that they have relatively large flow cross-sections, so that large particle-containing liquids can also be sprayed.
FIG. 1 shows in exemplified manner a prior art two-fluid nozzle 3 substantially symmetrical to axis 24. The liquid 1 to be sprayed is introduced into the mixing chamber 7 via a central lance tube 2 at the bottleneck 10. The pressure gas 15 is supplied by means of an external lance tube 4 to an annular chamber 6 surrounding in annular manner the mixing chamber 7. The pressure gas is introduced into the mixing chamber 7 by means of a certain number of holes 5. A first dispersion of the liquid in droplet form takes place in the mixing chamber 7, so that here a droplet-containing gas 9 is formed. There is also a bottleneck 14 at the exit from the mixing chamber 7. To the bottleneck 14 is connected a divergent exit 26, which terminates with the nozzle orifice 8. The droplet-containing gas flow 9 formed in the mixing chamber 7 is highly accelerated in the convergent-divergent or Laval nozzle, so that here a further droplet dispersion is brought about.
Two-fluid nozzles with a single exit hole of a conventional construction suffer from the fact that the jet 21 of droplets and atomization air passing out of the nozzle only has a limited opening or aperture angle α, so that relatively large distances or large containers are required for droplet evaporation.
In the case of such nozzles a fundamental problem arises due to the walls in the mixing chamber 7 being wetted with liquid. The liquid wetting the wall in the mixing chamber 7 is driven towards the nozzle orifice 8 as a liquid film by the shear and compressive stresses. An attempt is made to accept that the walls towards the nozzle orifice 8 are blown dry due to the high flow rates of the gas phase and that only very fine droplets are formed from the liquid film.
However, theoretical and experimental work carried out by the inventor has shown that liquid films can still exist on walls in stable film form without droplet formation if the gas flow driving the liquid film towards the nozzle orifice 8 achieves supersonic speed. This is the reason why it is possible to use a liquid film cooling in rocket thrust nozzles. The film flow is particularly critical when spraying high viscosity liquids, which simultaneously have a high surface tension, e.g. glycol in cryogenic dryers of natural gas pumping stations or solid suspensions in spray absorbers.
The liquid films driven by the gas flow to the nozzle orifice 8 can, as a result of adhesiveness, even migrate around a sharp edge at the nozzle orifice 8 and then form on the outside of the nozzle orifice 8 a water bulge 12. From the water bulge 12 are detached marginal droplets 13, whose diameter is a multiple of the average droplet diameter in the jet core. Although these large marginal droplets only contribute a small mass proportion to the droplet load, they are still determinative for the container dimensions, in which the temperature of a gas is to be lowered by evaporation cooling from 350° C. to 120° C., without there being an introduction of droplets into downstream components such as a fan or fabric filter.
The not previously published German patent application DE 10 2005 048 489.1 of the same inventor relates to a two-fluid nozzle, in which the formation of large marginal droplets is reliably prevented by annular clearance atomization. The content of the patent application is fully included by reference in the present application. FIG. 2 shows a corresponding two-fluid nozzle with annular clearance atomization. In the case of the variant shown the annular clearance air, also referred to as secondary air, is branched off directly from the annular chamber 6 via holes 19. However, this nozzle type also suffers from the property of producing a relatively slender jet 21 with an opening angle α of approximately 15°. It is known that such nozzles can fundamentally be surrounded by a screen or barrier air ring 25 and a screen or barrier air nozzle 23. The essential difference between barrier air 11 and annular clearance air is that the total pressure of the annular clearance air leaving the annular clearance 16 coincides from the order of magnitude standpoint with the pressure of the pressure gas 15 for atomization, whereas the pressure of the barrier air 11 is generally one or two orders of magnitude lower.
The pressure gas leaves the annular clearance 16 with a high velocity and ensures that a liquid film on the wall of the nozzle orifice 8, particularly of the divergent exit section, is drawn out to a very thin liquid lamella, which then is broken down into small droplets. This prevents or reduces to a tolerable level the formation of large droplets from wall liquid films in the nozzle exit area and at the same time the fine droplet spectrum in the jet core can be maintained without it being necessary for this purpose to increase the pressure gas consumption of the two-fluid nozzle or the energy requirements linked therewith. The annular clearance air quantity can e.g. be 10 to 40% of the total atomization air quantity. The total pressure of the air in the annular clearance is advantageously 1.5 to 2.5 bar absolute. The total pressure of the air in the annular clearance is advantageously so high that on expansion to the pressure level in the container the speed of sound is roughly reached. The exit opening is formed by a circumferential wall, whose outermost end forms an exit edge and the annular clearance is located in the vicinity of the exit edge. Appropriately the annular clearance is formed between the exit edge and an outer annular clearance wall. Considered in the outflow direction, the annular clearance wall edge is positioned downstream of the exit edge. Advantageously the annular clearance wall edge is positioned downstream of the exit edge by 5 to 20% of the exit opening diameter. A pressure of the pressure gas supplied to the annular clearance and a pressure of the pressure gas issuing through the pressure gas inlet into the mixing chamber 7 can be adjusted independently of one another. The inlet openings 5 into the mixing chamber 7 can be oriented tangentially to a circle about the nozzle median longitudinal axis, in order to produce an angular momentum in a first direction. Several inlet openings can be provided spaced from one another and different inlet openings can be so tangentially oriented that they produce an angular momentum in different directions, e.g. also opposing angular momentum directions.
By reference the content of the not previously published patent application DE 10 2006 001 319.0 is also completely included in the present application. Patent application DE 10 2006 001 319.0 describes a two-fluid nozzle for wall-bound installation, in which in order to avoid wall coatings an envelope, barrier or screen air nozzle and the wall area around the nozzle are heated. Otherwise the nozzle described therein is identical to the two-fluid nozzle according to DE 10 2005 048 489.1.
It is common to all the above-described two-fluid nozzles that the opening angle of a spray jet produced is comparatively small, so that long distances are required for droplet evaporation.
The present invention provides a two-fluid nozzle with which it is possible to obtain a large spray jet opening angle.
For this purpose, according to the invention a two-fluid nozzle is provided having a main nozzle, a mixing chamber and a nozzle orifice connected to the mixing chamber and positioned downstream thereof, in which secondary air nozzles issue in an annular manner in the vicinity of the nozzle orifice.
Through the provision of a ring of secondary air nozzles positioned in the vicinity of the nozzle orifice or also surrounding the nozzle orifice it is possible to produce a nozzle jet with a much greater opening angle α of at least approximately 30° to 45°. Compressed air jets passing out of the secondary air nozzles act on the jet of droplets and atomization air passing out of the nozzle and widen the same. At the same time and without a continuous annular clearance it is possible to retain the advantages of annular clearance atomization according to German patent application DE 10 2005 048 489.1 and specifically the formation of large marginal droplets is prevented.
Therefore the inventive nozzle results from a two-fluid nozzle with an annular clearance atomization according to the not previously published German patent application DE 10 2005 048 489.1, in that the annular clearance for annular clearance atomization is replaced by a ring of individual air nozzles which surround the nozzle orifice. Surrounding is here intended to mean that the individual secondary air nozzles are arranged in a circular manner around the nozzle orifice and that in the case of several secondary air nozzles their exit jets are in contact or even are superimposed in the vicinity of the nozzle orifice, so that a continuous annular secondary air jet surrounds the nozzle orifice. The imaginary projections of the secondary air holes in the plane of the nozzle orifice can be superimposed to a closed, annular surface. Thus, individual secondary air nozzle holes start in the comparatively wide annulus outside the mixing chamber, but during the further travel in the direction of the nozzle orifice can be in contact with each other or even overlap at the position of the latter. Besides the already discussed overlap of the projections of the extensions of the nozzle holes on the plane of the nozzle orifice it is naturally also possible to introduce secondary air holes that they are already superimposed in the vicinity of the exit and in the vicinity of the nozzle orifice, so that the nozzle orifice wall has an annular, circumferential recess. Thus, the inventive nozzle offers the possibility, as a function of the diameter or arrangement of the secondary air holes, of providing an annular clearance with a variable width. This is particularly important when manufacturing nozzle series or families if use is to be made of the same body with different annular clearance widths. Thus, the inventive nozzle can have a geometrical overlap of the secondary air holes in the vicinity of the nozzle orifice, and the overlap occurs either in the nozzle orifice wall area or only on an imaginary plane level at the height of the nozzle orifice. However, in addition to the secondary air nozzles, it is also possible to have an annular clearance atomization. Through the provision of annularly arranged secondary air nozzles, as a result of a redesign in the vicinity of the nozzle orifice, a two-fluid nozzle with internal mixing can be transformed into a nozzle with a wide angle jet.
According to a further development of the invention a main spraying direction of the secondary air nozzles is oriented into a main spray jet emanating from the nozzle orifice.
As a result of such a secondary air nozzle orientation, entry takes place into the main nozzle spray jet so as to widen the same.
According to a further development of the invention the median longitudinal axes of the secondary air nozzles are arranged under an angle β of 20° to 80° to a median longitudinal axis of the main nozzle.
Thus, the spray jet of the secondary air nozzles receives both a component parallel to the median longitudinal axis of the main nozzle and also a component perpendicular thereto and which is mainly responsible for widening the spray jet. Different widenings of the spray jet can be obtained by varying the angle β.
According to a further development of the invention the median longitudinal axis of the secondary air nozzles do not intersect the median longitudinal axis of the main nozzle.
As a result of a skewed arrangement of the median longitudinal axes of the secondary air nozzles it is possible to bring about a particularly uniform spray jet widening. With a corresponding arrangement of the secondary air nozzles an angular momentum can e.g. be impressed on the main nozzle spray jet which aids a widening of the jet.
In a further development of the invention the secondary air nozzles are oriented tangentially to an imaginary circle concentric to the median longitudinal axis of the main nozzle.
This makes it possible to obtain a very effective spray jet widening with fine droplet atomization. Considered in the direction of the median longitudinal axis of the main nozzle, the median longitudinal axes of the secondary air nozzles appear as tangents, which engage on an imaginary circle concentrically surrounding the main nozzle median longitudinal axis. As the secondary air nozzles also form an angle of less than 90° with the main nozzle median longitudinal axis, they are consequently in contact with an imaginary circular cylinder concentrically surrounding the main nozzle median longitudinal axis. Advantageously the imaginary circle has a radius between 30 and 80% of the radius of the main nozzle spray jet level with the circle. Such an orientation of the secondary air nozzles leads to a significant widening of the spray jet in the case of fine droplet atomization. On considering the imaginary circle with which there is tangential engagement of the projection of the median longitudinal axes of the secondary air nozzles and specifically the plane in which the circle is located, the plane forms with the outer border of the main spray jet a circular intersection with a spray jet radius. The imaginary circle then has a radius which is between 30 and 80% of the spray jet radius. Advantageously the imaginary circle is positioned downstream of the main nozzle orifice. The contacting of the median longitudinal axes of the secondary air nozzles consequently takes place on an imaginary circular cylinder around the main nozzle median longitudinal axis downstream of the nozzle orifice.
In a further development of the invention the secondary air nozzles open out or issue upstream of the main nozzle orifice into the outflow channel from the mixing chamber to the nozzle orifice.
It has proved advantageous if the secondary air nozzles open out or issue into the outflow channel directly upstream of the nozzle orifice. It can be advantageous for the orifices of the secondary air nozzles to contact or partly overlap at the entrance into the outflow channel.
In a further development of the invention there is a separate supply air line to the secondary air nozzles.
In this way the air quantity and the velocity of the air leaving the secondary air nozzles can be separately adjusted and e.g. used for setting a desired spray jet angle. For this purpose adjusting means are then required for adjusting the air pressure at the secondary air nozzles.
According to a further development of the invention the secondary air nozzles are in flow connection with a pressure gas supply line, which is also in flow connection with the mixing chamber.
A simple construction of the inventive nozzle is obtained if the air required for the secondary air nozzles is branched from the main nozzle pressure gas supply line. To this end, the secondary air nozzles can be connected to an annulus surrounding the mixing chamber. As a result the inventive two-fluid nozzle can have a very compact construction.
According to the invention the nozzle orifice is surrounded by an annular clearance, compressed air being supplied to the annular clearance.
Through the provision of such an additional annular clearance atomization water droplets at the nozzle orifice emanating from a liquid film covering the outflow channel wall can be drawn out to form liquid lamellas and atomized in fine droplet form. An additional annular clearance atomization can then be particularly advantageous if the individual secondary air nozzles are not in contact or overlap at the edge of the outflow channel.
According to a further development of the invention, starting from the mixing chamber, an outflow channel initially continuously narrows and then, starting from a bottleneck in the outflow chamber, then continuously widens towards the nozzle orifice.
As a result the two-fluid mixture passed through the outflow channel is highly accelerated in the convergent-divergent-nozzle and a fine droplet distribution in the spray jet can be obtained. The outflow channel can be so designed and the pressure of the liquid and pressure gas so adjusted that at least zonally supersonic speed is reached in the outflow channel.
According to a further development of the invention an additional screen air nozzle annularly surrounding the nozzle orifice is provided.
Such a screen or envelope air nozzle can be provided in addition to the annular clearance for annular clearance atomization and is supplied with screen air at a lower pressure than is required for annular clearance atomization.
Further features and advantages of the invention can be gathered from the claims, the following description and the drawings. Individual features of the different embodiments of the invention shown in the drawings can be randomly combined without passing beyond the scope of the invention. In particular, the features of the two-fluid nozzle shown in FIG. 2 can be randomly combined with the nozzles shown in FIGS. 3, 4 and 5 without passing beyond the scope of the invention.